(1) Field of the Invention
The present invention generally relates to malfunction detection in an evaporated fuel purge system, and more particularly to an apparatus for detecting a malfunction in an evaporated fuel purge system used in an internal combustion engine.
(2) Description of the Related Art
An evaporated fuel purge system is used in an internal combustion engine. In the evaporated fuel purge system, fuel vapor fed from a fuel tank is absorbed in an absorbent in a canister, and the fuel vapor is prevented from escaping to the atmosphere. This system functions to feed the absorbed fuel into an intake passage of the engine when the engine is in a prescribed operating condition and the intake passage is subjected to a negative pressure below the atmospheric pressure.
The evaporated fuel purge system (hereinafter called the system) includes a vapor passage for connecting the fuel tank to the canister and a purge passage for connecting the canister to the intake passage of the engine. There is provided a purge control valve for controlling a flow of fuel vapor within the system. When the vapor passage or the purge passage is damaged due to a certain problem, or when a connecting pipe is disconnected from either of the passages, the fuel vapor may escape from the system to the atmosphere and a serious problem may arise in the engine. When the purge passage between the canister and the intake passage clogs due to a certain problem, the fuel vapor in the canister may overflow, so that fuel vapor may leak from an air inlet hole of the canister to the atmosphere. Thus, it is necessary to diagnose malfunctions in the evaporated fuel purge system, in order to perform desired operations of the engine.
There have been proposed several malfunction detecting devices for diagnosing the system used in the engine. One of the devices is disclosed in U.S. patent application Ser. No. 895,102 filed on Jun. 8, 1992 (assigned to the applicant of the present invention). This malfunction detecting device includes a first control valve for controlling a flow of fuel vapor in the purge passage between the canister and the intake passage, and a second control valve for opening and closing the air inlet hole of the canister, the hole communicating with the atmosphere. When the system is diagnosed, the second control valve is closed, and the first control valve is also closed when a prescribed negative pressure level is sensed in the fuel tank. The level of the negative pressure in the fuel tank is maintained for a prescribed time period. It is detected whether a malfunction occurs in the system in response to a level of pressure change in the fuel tank during the time the level of the negative pressure is maintained.
However, in the above mentioned device, when a relatively large leakage occurs in the system, it takes a long time for the level of tank internal pressure to reach a prescribed initial negative pressure level. If the initial negative pressure level is not reached due to occurrence of a considerable leakage, the system is not correctly diagnosed.
In the prior art, there is another malfunction detection device for diagnosing the system. This device has the construction similar to that of the aforementioned device. The malfunction detection device mentioned above merely checks whether or not a reference level of negative pressure is reached in the system after the second control valve is closed. It is detected whether a leakage occurs in the system depending on whether the reference level of the negative pressure is sensed. In a case of the above mentioned device, it is difficult to correctly detect a slight leakage in the system by sensing the reference level of negative pressure, because the flow of fuel vapor in the system varies somewhat considerably and the fuel within the fuel tank always evaporates.
Other disadvantages or problems of the above mentioned devices and other conventional devices will become more apparent from the detailed description of preferred embodiments of the present invention when read in conjunction with the accompanying drawings.